There has been conventionally proposed an injection molding device which supplies a heat medium such as steam to heat a mold, thereby increasing the temperature thereof to a target temperature (refer to FIG. 1 in Patent Document 1). In this injection molding device, an ordinary discharge line 13 is connected to a heat medium discharge port of a mold 1, and a steam trap 12 is installed on the ordinary discharge line 13. There is also installed an outlet bypass line 15 which bypasses the steam trap 12 and is communicatively connected to the upstream side and the downstream side of the ordinary discharge line 13. The outlet bypass line 15 is installed so as to bypass the ordinary discharge line 13, and a transfer valve 14 is installed on the outlet bypass line 15. Here, a discharge rate adjusting unit which is constituted with the steam trap 12, the outlet bypass line 15 and the transfer valve 14 is mounted on a mold temperature adjustor which is separately provided from the mold 1.
Then, in the above-constituted conventional injection molding device, during the first half of a heating step of the mold 1, the transfer valve 14 is opened to distribute steam in a large quantity into the mold 1, thereby rapidly heating the mold. On the other hand, during the second half of the heating step, the transfer valve 14 is closed to keep the inside of the mold 1 high in pressure by causing the steam to pass through the steam trap 12. In addition, the steam is controlled for pressure to adjust the temperature of the mold with high accuracy.